In the production of today's integrated circuits, optical lithography is one of the key techniques. A well-known problem in optical lithography, related to the transparency of state-of-the-art resists, is the occurrence of unwanted multiple interference effects in the resist layer during illumination, caused by a relatively high substrate reflection. Light, which propagated through the resist, is partially reflected back into the resist by the substrate on which this resist has been deposited. The substrate itself can comprise a stack of various layers (e.g., a stack of dielectric layers or conductive layers formed on a semiconductor substrate).
Multiple interference effects result in a variation of illumination intensity with resist depth, causing a variation of the development rate with resist depth. As a result, the resist walls have a scalloped profile, or so-called ‘standing waves.’ In extremes, this standing wave problem will cause pattern collapse of lines in defocus, e.g., strongly pronounced standing waves at the bottom of the resist layer and/or incomplete development of lines or contact holes, especially in defocus.
The multiple interference effects in the resist will result in a variation of total absorbed energy with resist thickness, hence, in a variation of the critical dimension (CD) with resist thickness. The latter is known as the ‘swing effect,’ which will cause CD non-uniformity if patterns have to be made on substrates with topography.
These multiple interference effects in the resist and, hence, the total amount of energy absorbed will also depend on the layers underneath the resist and variations thereof in thickness and/or composition. For example, the interference effects will be different if a contact hole is to be printed in a resist layer on top of an oxide or on top of a nitride layer, as both dielectric layers have different optical properties, such as transparency.
Typically, bottom anti-reflective coatings (BARC), also referred to as bottom anti-reflective layers (BARL), are used in between the resist and the substrate to decrease the occurrence of multiple interference effects due to reflection by the substrate. Using such layers, the reduction in substrate reflectivity can take place in two ways: (1) by absorption of light in the BARC or (2) by destructive interference of light rays at the bottom of the resist. Often the use of a BARC is crucial to control the reflection caused by the substrate.
Conventionally, BARC thickness optimization is carried out by calculating the substrate reflectivity versus BARC thickness for light rays perpendicularly incident on the wafer. Such calculations typically can be performed by state-of-the-art lithography simulation programs or tools calculating basic optics. Typically, the substrate reflectivity will drop with BARC thickness due to absorption, but local minima and maxima in the substrate reflectivity curve as function of the BARC thickness are present due to interference effects. The first minimum of this curve that provides sufficiently low substrate reflectivity is considered to be the optimum BARC thickness.
In order to evaluate whether the lithographic process will lead to sufficient print quality, typically in conventional lithographic processes, the substrate reflectivity is considered sufficiently low when it is below 0.5%. BARC layers thus will be considered appropriate if the resulting substrate reflectivity is below this fixed, pre-determined value.